Many of the pathogenic protozoa are obligate intracellular parasites; they develop only within another living host cell. Examples are the human parasite Toxoplasma,sometimes responsible for serious congenital infection and recently of increased significance in AIDS patients, the cattle parasite Theileria parva, cause of East Coast fever and, most important, the malarial parasites. We would like to know the nature of this dependence on a living host cell. Has the parasite lost or suppressed certain biosynthetic capabilities, in this way gaining an energetic advantages? Does the host cell supply triggers for differentiation, in addition to essential nutrients? How much of the integrity of the host cell does the parasite require? One approach toward answering these and related questions is to attempt to replace the living host cell with an environment in which the parasite will develop extracellularly. Such initial extracellular development has been obtained with the invasive stages, the merozoites, of Plasmodium falciparum, the major human malarial parasite. In a medium based on concentrated red cell extracts, and supplemented with ATP and pyruvate, up to 20% of the erythrocytic merozoites show initial development into early rings, their viability indicated not only by their appearance in stained films but also by their uptake of Rhodamine 123. Studies are proposed to extend and improve upon this development. The extracellular forms differ from young rings of the same age that have been lysed out from their host erythrocytes in that they lack a parasitophorous membrane. Comparative studies of these forms at the ultrastructural level and with the use of monoclonal antibodies to the ring stage now developed will provide information on the nature of the parasitophorous membrane. The induction and development of gametocytes will also be studied with special reference to the possible role of protein kinases and to the basis for the recently discovered greater formation of gametocytes in younger erythro- cytes. Results of the proposed work will contribute to understanding the physiology of the erythrocytic stages of malarial parasites and will bear on fundamental problems in intracellular parasitism.